After an engine cold-start, exhaust emissions and fuel consumption tend to be higher. This is because cold combustion surfaces lead to poor fuel evaporation, resulting in fuel films surviving on combustion surfaces even after the combustion event has occurred. The fuel-rich area above the film, and the fuel evaporating from the film after the flame has passed can lead to increased hydrocarbon and particulate matter (PM) emissions. In addition, cold engine oil leads to increased friction losses.
In recent years, spark ignited combustion engines have been configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines can vary the effective displacement of the engine by skipping the delivery of fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern. For example, as shown by Tripathi et al. in U.S. Pat. No. 8,651,091, an engine fuel controller may continuously rotate which particular cylinders are fueled, which cylinders are skipped, and how many cylinders events the pattern is continued for. In addition, individual valve mechanisms of each cylinder may be selectively deactivated. By skipping fuel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine.
The inventors herein have recognized that the individually controllable valves of the selectively deactivated cylinders may be leveraged to improve the rate of cylinder warming over a drive cycle. In particular, valve operation of a cylinder that will not be fired on the next cycle due to engine load may be controlled to retain hot exhaust gases in the cylinder, thereby heating the combustion chamber surfaces faster. In one example, some of the above issues may be addressed by a method that increases the rate of engine warming during cylinder deactivation comprising: selecting a cylinder for deactivation; and on a combustion cycle immediately preceding the deactivation, maintaining an exhaust valve of the selected cylinder closed during an exhaust stroke of the cylinder. In this way, the fast-responding individual cylinder valve mechanisms can be leveraged to increase combustion surface temperatures.
As an example, in response to a drop in engine load, an engine controller may select a cylinder pattern of individual cylinders of mechanisms for selective deactivation. Therein, the controller may select a number and identity of cylinders to be deactivated. A cylinder that is selected for deactivation and that will not be fired on the next engine cycle may have its exhaust valve held closed during an exhaust stroke of the firing cycle immediately preceding the deactivation. Specifically, instead of exhausting the burned gases, the hot gases are retained in the cylinder by not opening the exhaust valve. The exhaust gases are then retained in the cylinder while the cylinder is deactivated on the next cycle.
In this way, by keeping the hot exhaust gases in the cylinder for the next cycle, the combustion chamber surface is heated much faster. Consequently, the entire engine is heated faster as well. Further, keeping the burned gases in the deactivated cylinder longer improves continuing oxidation of the hydrocarbons remaining in the combustion chamber, leading to improved emissions for that cycle. Overall, engine performance is improved.
Still other valve operations may be used in various combinations to improve engine heating and allow for expedited catalyst warming. For example, a cylinder to be deactivated may be fired normally and exhausted normally on the engine cycle preceding the deactivation. Alternatively, the cylinder may be configured to induct, but not fuel, and exhaust fresh charge. In still another example, the cylinder may induct and fire but not exhaust. By deciding to induct and pass through air, the exhausted air can be combined with exhaust from other cylinders running slightly rich to provide fuel and air at the exhaust catalyst. The reaction of the fuel and air at the exhaust catalyst generates heat that results in a fast catalyst light-off. Other alternatives may include inducting air and fueling on induction or compression strokes but not sparking to provide a fuel air mixture to the catalyst. Further still, the air may be inducted into a cylinder, compressed, but not fueled until the exhaust stroke, that is, as a post injection. This latter approach may also include sparking near the exhaust stroke and burning into the exhaust phase, to provide heat flux to the exhaust catalyst. In this way, the various combinations of firing, not firing, firing during post injection, running rich in some cylinders while pumping air with other cylinders, etc., may be used in various combinations, all while putting known torque pulses into the engine system in such a manner as to provide acceptable NVH characteristics. Additionally, cylinders that have been combusted normally (active cylinders) may be operated at a higher load making them more stable and tolerant of additional spark retard. The additional spark retard maybe advantageously used during these conditions to add more heat flux to the engine and the exhaust catalyst.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.